U.S. patent application number 10/374674 was filed with the patent office on 2004-04-01 for method of fabricating metal parts of different ductilities.
This patent application is currently assigned to Benteler Automobiltechnik GmbH. Invention is credited to Boke, Johannes, Krogmeier, Jurgen.
Application Number | 20040060623 10/374674 |
Document ID | / |
Family ID | 7713947 |
Filed Date | 2004-04-01 |
United States Patent
Application |
20040060623 |
Kind Code |
A1 |
Boke, Johannes ; et
al. |
April 1, 2004 |
Method of fabricating metal parts of different ductilities
Abstract
Hardened metal articles such as slabs, plates or sheets or
preformed metal articles which are to have regions of different
ductility are subjected to heating to an austenization temperature
and then to quenching of the region of greater ductility to form a
start temperature lying above the gamma-alpha transformation
temperature of the workpiece structure. The quenching is terminated
at a stop temperature lying above the martensitic temperature and
prior to any substantial transformation into ferritic or perlitic
structures. The higher ductility region is then maintained prior to
any substantial transformation into ferritic or perlitic
structures. The higher ductility region is then maintained
isothermically for condition of austenitite ferrite and/or perlite.
The lower ductility structure is brought to a temperature
sufficient for martensitic formation and the hardening is then
carried out.
Inventors: |
Boke, Johannes; (Aerzen,
DE) ; Krogmeier, Jurgen; (Hovelhof, DE) |
Correspondence
Address: |
THE FIRM OF KARL F ROSS
5676 RIVERDALE AVENUE
PO BOX 900
RIVERDALE (BRONX)
NY
10471-0900
US
|
Assignee: |
Benteler Automobiltechnik
GmbH
|
Family ID: |
7713947 |
Appl. No.: |
10/374674 |
Filed: |
February 26, 2003 |
Current U.S.
Class: |
148/641 |
Current CPC
Class: |
C21D 2211/008 20130101;
C21D 9/46 20130101; C21D 2221/00 20130101; C21D 1/19 20130101; C21D
2211/005 20130101; C21D 1/673 20130101; C21D 2211/009 20130101 |
Class at
Publication: |
148/641 |
International
Class: |
C21D 006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2002 |
DE |
10208216.2 |
Claims
We claim:
1. A method of producing a hardened metal article with at least two
regions of different ductility including at least one first region
with higher ductility and at least one second region of lower
ductility, comprising the steps of: (a) heating a metal slab or
preformed metal part constituting a workpiece to an austenitization
temperature; (b) thereafter transporting said workpiece over a
transport path and during transport of the workpiece along said
path subjecting said first region to cooling by: (b1) quenching
said first region from a predetermined cooling start temperature
(.sub.Tstart) lying above a .gamma.-.alpha. transformation
temperature of the workpiece, (b2) terminating the quenching when a
predetermined cooling stop temperature (.sub.Tstop) is reached
which lies above the martensitic start temperature and prior to any
transformation into ferritic or perlitic structures or prior to any
but a slight transformation into ferritic or perlitic structures,
and (b3) then maintaining the workpiece approximately under an
isothermal condition for conversion of austentite in the structure
of the workpiece to at least one structure selected from the group
consisting of ferrite and perlite; (c) during step (b) maintaining
said second region at a hardening temperature (T.sub.H) at least
sufficient for martensite formation in said second region; and (d)
then effecting a hardening process for said workpiece during
transport thereof along said path.
2. The method defined in claim 1 wherein said second region is
brought to said hardening temperature (T.sub.H) during step (b3)
which is less than the heating temperature in step (a).
3. The method defined in claim 1 wherein said second region is
subjected to heating to maintain an austentite structure
therein.
4. The method defined in claim 1 wherein heating radiated from said
second region is captured by a reflecting mirror and reflected back
to said second region.
5. The method defined in claim 1 wherein said first region is
cooled in step (b1) by a nozzle directing a cooling medium onto
said first region and matched to the geometry of the workpiece.
6. The method defined in claims 5 wherein said cooling medium is
air.
7. The method according to claim 1 wherein the hardening process is
carried out in a cooled reshaping tool in the course of a hot
forming of the workpiece.
8. The method defined in claim 1 wherein said first and second
regions are subjected to processes separated by a partition from
one another.
9. The method defined in claim 1 wherein the workpiece is composed
of a steel alloy containing manganese and baron components.
10. The method defined in claim 9 wherein the workpiece is
comprised of a steel alloy consisting, in weight percent,
essentially of: carbon (C) 0.18% to 0.3% silicon (Si) 0.1% to 0.7%
manganese (Mn) 1.0% to 2.50% phosphorus (P) maximum 0.025% chromium
(Cr) 0.1% to 0.8% molybdenum (Mo) 0.1% to 0.5% sulfur (S) maximum
0.01% titanium (Ti) 0.02% to 0.05% boron (B) 0.002% to 0.005%
aluminum (Al) 0.01% to 0.06% balance iron and unavoidable smelting
impurities.
Description
FIELD OF THE INVENTION
[0001] Our present invention relates to a method of making hardened
metal parts, especially motor vehicle components, having regions of
different ductility. More particularly, the invention relates to a
method whereby a workpiece, such as a slab, a plate or a preformed
metal part, usually of an alloy steel, is subjected to heating to
an austenitization temperature and then subjected to a hardening
process while being passed along a transport path and such that the
end product will have at least one region of higher ductility and
at least one region of lower ductility.
BACKGROUND OF THE INVENTION
[0002] It is known to produce shaped articles or vehicle components
which are hardened in the die or shaping tools. These can include
steering or cross bars or structural components like door impact
beams, B-columns, struts or shock-absorbing portions of the chassis
or vehicle body. These usually have uniform properties through the
shaped component and are produced using a complete hardening of the
component or by subjecting the entire component to annealing or
tempering processes. These parts then generally have high strength
and can retain their stability in the case of a crash.
[0003] However, it may be desirable to allow the components to be
deformable in the case of a crash and thereby dissipate the crash
energy as deformation energy. In certain applications in motor
vehicle construction, the shaped articles are required to have
certain regions of high strength and low deformability and other
regions which should be of greater ductility. For example, in the
case of B columns, the column foot should be relatively ductile
while the upper part of the column should have greater strength and
lower ductility. Apart from reinforcing the column portions which
are to have reduced deformability by joining additional members to
them or attaching reinforcing plates or the like, it is also known
to subject a component to such heating treatment that it will have
local regions of higher strength and thus lower ductility and
regions of higher ductility or deformability.
[0004] Thus the German patent document DE 197 43 802 C2 describes a
method of making a shaped structural component for a motor vehicle
with regions of different ductility in which a starting Blab or
billet prior to or after pressing is only partially heated or is,
after a prior homogeneous heating, is heated further in regions in
which the desired higher ductility is to be produced. The
subsequent heating to generate ductile regions can result in
distortion of the article.
[0005] German Patent document DE 197 23 655 A1 describes a method
of partially hardening a shaped structural component whereby the
starting slab is homogeneously heated in a furnace and then is
hardened in a cooled pair of dies whereby partial regions of the
workpiece are subjected to hardening with slow cooling in that, at
these locations in the tools, recesses or thermally insulating
inserts are arranged or inductive heating is effected at these
regions. The purpose of this process is to enable a workpiece to be
machined additionally in the partially nonhardened regions, for
example by boring. The method of DE 197 23 655 A1 however has
problems in the case of hot-forming processes since the shaping
cannot occur in the regions in which recesses are provided in the
tools and in which greater ductility is to be provided by
preventing or limiting the hardening. As a consequence breakage can
occur. The inductive hardening is only possible for the finally
shaped parts and requires a separate process step. As a consequence
the subsequent inductive hardening is expensive and can involve the
danger of distortion.
[0006] European Patent EP 0 816 520 B1 describes a shaped article
and a method for creating defined strength and hardness
characteristics thereof over its length whereby the article after
shaping is inductively heated and then quenched to produce hardened
regions.
[0007] DE 200 14 361 U1 describes a B column which also has regions
of different strength. The production of the B column is effected
by a hot forming process in which a steel blank or preformed
elongated section is austenitized in a furnace and then shaped and
hardened in a cooled die. In the furnace, large-area regions of the
workpiece can be shielded against the effect of the temperature by
insulation so that in the shielded regions the austenitization
temperature is not reached and as a consequence in the workpiece
there is no martensitic structure upon hardening.
[0008] Alternatively, it is proposed to subject the steel section
completely to austenitization and during the transport to bring a
region to a temperature significantly below the austenitization
temperature, for example by blowing onto this region for a cooling
at a limited or slow rate within the hardening tool. In the
hardening tool, therefore, no pure martensitic structure arises but
rather a mixed structure with clear ferrite/bainite structures is
produced which has ductile characteristics.
[0009] This process has several problems in its practical
application in mass production. The use of insulation of shielding
in the furnace is itself an expensive operation since in each cycle
individual parts must be insulated separately. The insulation must
be provided in a preparation stage and increases the duration of
the heating process and the insulation, in the case of re-use must
be heated up. This makes mass production expensive. A cooling which
is intended not to be too sharp is difficult to control, especially
when it is intended to bring the temperature to a point
significantly below the austenitization temperature and is a
problem for mass production systems. The products which are
produced may have variable properties as a consequence.
OBJECTS OF THE INVENTION
[0010] It is, therefore, the principal object of the present
invention to provide a method of making metal products, especially
vehicle structural components, with at least two regions of
different ductility, which is suitable for mass production.
[0011] Another object is to provide a method for the purposes
described which avoids the problems hitherto encountered in
fabricating steel articles with regions of different ductivity.
SUMMARY OF THE INVENTION
[0012] These objects are achieved, in accordance with the
invention, in a method of producing a hardened metal article with
at least two regions of different ductivity including at least one
first region with higher ductivity and at least one second region
of lower ductivity. The method comprises the steps of:
[0013] (a) heating a metal slab or preformed metal part
constituting a workpiece to an austenitization temperature;
[0014] (b) thereafter transporting the workpiece over a transport
path and during transport of the workpiece along the path
subjecting the first region to cooling by:
[0015] (b1) quenching the first region from a predetermined cooling
start temperature (T.sub.start) lying above a .gamma.-.alpha.
transformation temperature of the workpiece,
[0016] (b2) terminating the quenching when a predetermined cooling
stop temperature (T.sub.stop) is reached which lies above the
martensitic start temperature and prior to any transformation into
ferritic or perlitic structures or prior to any but a slight
transformation into ferritic or perlitic structures, and
[0017] (b3) then maintaining the workpiece approximately under an
isothermal condition for conversion of austenite in the structure
of the workpiece to at least one structure selected from the group
consisting of ferrite and perlite;
[0018] (c) during step (b) maintaining the second region at a
hardening temperature (T.sub.H) at least sufficient for martensite
formation in the second region; and
[0019] (d) then effecting a hardening process for the workpiece
during transport thereof along the path.
[0020] In step (a), therefore, the metal slab or preformed metal
parts is brought in a heating device to a defined austenitization
temperature for a predetermined austentitization time and thus is
homogeneously heated to a temperature which can correspond to the
cooling start temperature.
[0021] By contrast with a continuous cooling of the first regions
at a low cooling rate, the invention provides in its first step for
a rapid quenching of the first regions to a cooling stop
temperature or transformation temperature and then a substantially
isothermal transformation into a ferritic/perlitic structure. This
has the advantage that an exact setting of the transformation
temperature parameter and the retention time parameter for the
ferritic/perlitic structure component can be readily set and
controlled and thus that the mechanical characteristics are
controllable in a highly reliable manner. It is also of advantage
that the parallel processes for creating the ductile
characteristics of the first regions and the process for creating
the low ductility high strength second regions have identical
process commencement and the same terminations and hence the same
process times. The method can thus be integrated in a problem-free
manner in already existing hot-forming processes.
[0022] In an alternative, the quenching step can commence at a
higher cooling speed which is greater than the critical cooling
speed, i.e. the cooling speed at which a ferritic/perlitic
structure is formed and which can be halted at a precisely
determined temperature. This temperature is so chosen that it is a
maximum for the ferritic/perlitic transformation at the highest
possible rate and simultaneously is a compromise. At lower
temperatures, the transformation of the austenite is greater but
the increasing diffusion inertia of the carbon atoms delays the
process. In contrast thereto the diffusion of the carbon atoms is
significantly greater at higher temperatures but the transformation
of the austenite is much less. The duration of the retention time
required for structure transformation also has the direct influence
on the amount of the remaining residual austenite content in the
first regions.
[0023] Since this retention time cannot optionally be increased in
the case of mass production and the hardening temperature for the
second regions optionally lowered, a fairly precise agreement
between the different cooling processes to which a workpiece must
be subject is required. The optimization of the temperatures and
retention times ensures ductile and high strength regions in a
single structural component.
[0024] During the isothermal transformation in the first regions,
the second regions are predominantly or completely maintained in
the austenitic range. As a result it is of special advantage to
match the transformation time span with the austenitization
temperature selected in the heating furnace such that the hardening
temperature for the second regions over the transformation time is
less than the heating temperature in the furnace.
[0025] It is especially advantageous and an optimal match for the
hardening temperature to be so high that a martensite formation in
this region occurs during the hardening process. Preferably an
excessive temperature drop in the second region can be counteracted
by a supply of heat thereto during the transformation of the first
region. It can be sufficient, in this case, to avoid radiation loss
from the second region or to minimize radiation loss, for example
by reflecting radiation back onto the second regions.
[0026] In order to ensure that the rapid cooling process and the
isothermic stage are reproducible, the first regions are cooled, in
accordance with the invention, with a cooling medium dispensed from
nozzles conforming to the geometry of the first regions of the
workpiece. The cooling medium is preferably an air stream.
[0027] The hardening process can be carried out in any optional
hardening device, for example, in a quenching vessel. It is however
especially advantageous to effect the quenching using a cooled
tool, for example, a shaping die in conjunction with a shaping
operation. This mode of operation has been found to be especially
effective when the process is part of a hot-forming process. In
that case the hardening step, which involves quenching below the
martensitic starting temperature or forming martensite in the
austenitic structure of the second regions, is effected in contact
with the cooled die. Additional steps such as tempering or
annealing can follow. The result is a continuous rather than an
abrupt transition from the more ductile structure to the harder
structure between the first and second regions.
[0028] In addition to air nozzles conforming to the geometry of the
workpiece for local cooling of the first regions, it is
advantageous to shield the processes in the two regions from one
another, for example by a partition in the form of a sheet or
plate. This permits the transition from first regions of high
ductility to regions with higher strength to be established with
precision.
[0029] A precipitous transition from ductile to the high strength
over a small transition region can thus be obtained if desired or a
transition region which is wide and gradual can be created with the
material characteristics running from ductile to high strength or
vice versa depending upon the desire of the operator.
[0030] The method is particularly suitable for use with steel
alloys containing manganese and boron. With such steels the
critical cooling speed, i.e. the cooling speed which a martensitic
structure arises is significantly reduced. The boron addition
results, during the cooling of steel in a delay of the
transformation into softer structural types like ferrite and
perlite starting from the austenitic range. This means that slower
cooling speeds can produce a hardening in the material like that
which can be achieved with a continuous air stream. These steel
types have been hardened in according with the German patent
document DE 200 14 361 U1 using an air stream over the entire
structure of the workpiece and will not yield ductile regions.
[0031] The invention is preferably applied to a slab of a steel
alloy having, in weight percent, carbon between 0.18% and 0.3%,
silicon between 0.1% and 0.7%, manganese between 1.0% and 2.5%,
phosphorus to a maximum of 0.025%, chromium from 0.1% to 0.8%,
molybdenum between 0.1% and 0.5%, sulfur to a maximum of 0.01%,
titanium between 0.02% and 0.05%, boron between 0.002% and 0.005%
and aluminum between 0,01% and 0.06%, the balance being certain
unavoidable smelting impurities. Although not mandatory the steel
alloy can have a niobium content (Nb) between 0.03% and 0.05% to
minimize intercrystalline corrosion and resistance to heat.
[0032] The method of the invention with the described interrupted
quenching step and the isothermal retention at a temperature above
the martensitic start temperature in the case of boron and
manganese-containing steel ensures ferrite/perlite transformation
for a softer structure in the first regions of the workpiece.
Because of the presence of boron, it is possible to provide a
reduced hardening temperature in the second regions so that during
the retention time a harder structure is ensured with the requisite
higher strength.
BRIEF DESCRIPTION OF THE DRAWING
[0033] The above and other objects, features, and advantages will
become more readily apparent from the following description,
reference being made to the accompanying drawing in which:
[0034] FIG. 1 is a schematic diagram of the method of the
invention;
[0035] FIG. 2 is a graph of temperature vs. time illustrating the
transformation start and end points and the times at which they
occur; and
[0036] FIG. 3 is a positive view illustrating aspects of the
invention and in particular the cooling of the workpiece with a
partition or shielding between process zones.
SPECIFIC DESCRIPTION
[0037] FIG. 1 shows the process sequence in the production of
structural components for a motor vehicle having regions of
different ductility. The fabrication line comprises a heating unit
1 or furnace in which the slab, plate or sheet 2, or a preformed
component, is homogeneously heated over a certain austenitization
time t.sub.a to a predetermined austenitization temperature
T.sub.A. On the transport path to a hardening unit 3, for example,
a reshaping die and press, in which the slab is then subjected to
shaping under uniform cooling, the process is subdivided into two
process stages P1 and P2 in which the local processing of different
regions of the workpiece enables the creation of different
deformation properties in the workpiece which will remain in the
finished product.
[0038] Between the heating stage 1 and the hardening stage 3, in a
process line P1 in which the more ductile regions are to be formed
in the first regions of the finished product, there is, for
example, a heating bed wherein, for instance, in case the intrinsic
heat of the component is not sufficient, hot air is blown therein.
The zone for maintaining the austenitic region 6 of the second or
less ductile part in the second process line P2 is preferably
provided with an additional heating device 7, for example, having
induction coils. The radiant heat from this second part of the
workpiece can also be reflected back onto the workpiece by means of
a mirror or other reflective unit 8.
[0039] When the component is a previously fabricated component, for
example, a B-column, the column after heating in the furnace is
displaced with its longitudinal axis traverse to the transport
direction on a conveyer belt which is represented by the paths P1
and P2 between the furnace 1 and the die 3. The column foot is
initially rapidly cooled (quenched) at 4 and then over the stretch
5 held isothermally while the structure of the workpiece in the
upper column part by transport through the zone 6 is maintained in
the austenitic range. Then, the component is subjected to hardening
and shaping in the cooled dies 3.
[0040] The temperature course of the two partial process lines P1
and P2 has been represented in FIG. 2. Starting from the common
austenitization temperature T.sub.A, the first regions, which are
to be softer in the finished product and thus to have a more ducted
structure, are brought from the cooling start temperature
(.sub.Tstart), which here corresponds to the austenitization
temperature, at the time t.sub.1 with a cooling or quenching rate
of 100 to 200 k/s to the cooling stop temperature (.sub.Tstop) or a
transformation temperature of 400.degree. C. to 800.degree. C. at
the time t.sub.2. The first regions are then subjected to
isothermal transformation approximately at this temperature to the
time t.sub.3. During this period, the second regions which are to
have a structure with reduced ductility in the final product are
maintained in the austenitic range until the transformation of the
structure of the first regions has been concluded or is nearly
concluded. At the time T.sub.3, the hardening process occurs in
which both regions are quenched. The first regions are quenched
from the temperature T.sub.stop while the second regions are
quenched from the hardening temperature T.sub.H.
[0041] FIG. 3 shows, in a perspective view, a shaped component 9
with a ductal region 10, referred to herein as the first region,
and a low ductility high strength region 11, referred to as the
second region. The transport direction is represented by the arrow
A.
[0042] The two regions 10 and 11 can be separated by a sheet or
plate 12 forming a partition between the process zones of the
workpiece as it passes along the transport path through the process
stages P1 and P2. The partition 12 matches the shape of the
workpiece 9, The region 10 which is to be more ductile in the
finished product, is juxtaposed above and below with nozzles 13,
13a, 13b, 13c, 13d and 13e which are shaped to conform to the
contour of the workpiece and inform which air is directed at the
workpiece to effect the quenching or rapid cooling defined in FIGS.
1 and 2. The cooling medium may be air. During this cooling part 11
which is to be less ductile and of higher strength in the finished
product is not subjected to cooling and indeed is protected by the
partition 12 from cooling.
[0043] The resulting part, when finally hardened, has regions with
two different structures and ductility and the corresponding
different mechanical properties. The particular temperatures and
times used can be matched to the different alloying elements and
compositions employed and the method has been found to be
applicable to components having large regions of high ductility
while avoiding problems hitherto encountered like distortion and
the requirements for extra steps.
[0044] A suitable composition in accordance with the invention and
provided as an example is a manganese-boron steel alloy of the
following composition (in weight %):
[0045] carbon (C) 0.18% to 0.3%
[0046] silicon (Si) 0.1% to 0.7%
[0047] manganese (Mn) 1.0% to 2.50%
[0048] phosphorus (P) maximum 0.025%
[0049] chromium (Cr) 0.1% to 0.8%
[0050] molybdenum (Mo) 0.1% to 0.5%
[0051] sulfur (S) maximum 0.01%
[0052] titanium (Ti) 0.02% to 0.05%
[0053] boron (B) 0.002% to 0.005%
[0054] aluminum (Al) 0.01% to 0.06%.
* * * * *